The present invention relates to a method and an apparatus for controlling a power transmission system for a motor vehicle.
A power transmitting system for a vehicle including a continuously variable transmission (CVT) is provided with an auxiliary transmission for changing a direction of drive of the vehicle and for improvement in drivability.
In the art of motor vehicles, there is known a power transmitting system incorporating a continuously variable transmission, and an auxiliary or secondary transmission which is connected in series with the continuously variable transmission and which has at least two forward drive shift positions. In this type of power transmitting system, the continuously variable transmission may be designed with a relatively narrow range of speed ratio, and may be accordingly small-sized. In addition, the continuously variable transmission may operate to transmit a relatively large quantity of drive torque without slippage of the belt on the pulleys, when the auxiliary transmission is disposed on the output side of the variable transmission. An example of such type of transmitting system is disclosed in Japanese Patent Application No. 58-144985 assigned to the assignee of the present application.
Generally, the shifting operation of the auxiliary transmission is automatically achieved according to predetermined shift-down and shift-up patterns, based on an opening of a throttle valve and a running speed of the vehicle. The overall speed ratio of the power transmitting system is changed with a change in the speed ratio of the continuously variable transmission. Hence, the condition in which the auxiliary transmission is shifted cannot be constant, and the transmitting system is inevitably subject to a shifting shock of varying magnitude upon shifting operation of the auxiliary transmission. Further, rotating members of the continuously variable transmission, such as variable-diameter pulleys, will have a comparatively large moment of inertia. Thus, the power transmitting system may have a considerable amount of shifting shock when the auxiliary transmission is operated across the different shift positions, whereby the driving feel of the vehicle is adversely influenced.
Similar inconveniences are encountered in a power transmitting system wherein the auxiliary transmission is shifted in response to the operation of an operator-controlled manual shift lever. That is, the transmitting system may have a shifting shock of varying and large magnitudes, due to a change in the speed ratio of the continuously variable transmission, and its relatively large moment of inertia.